Claims. (Cl. 166-35) The present invention is directed to a method for completing oil and gas wells. More particularly, the invention is directed to a method for completing oil and gas wells in which a drilling fluid or mud is used to control the well. In its more particular aspects, the invention s directed to a method for completing an oil or gas well 1n which a column of drilling mud is maintained in a casing to control the well.

Current practice when completing wells, such as oil and gas wells, through perforated casings is to have drilling fluids, such as mud, salt Water, water, or oil, in the well casing and to perforate the casing with bullet, shaped charge, chemical or punch-type perforators. When the pressure of a formation traversed by the well exceeds the hydrostatic pressure of a column of oil or Water at the completion depth, it is customary to use weighted salt water or normal drilling mud having a density great enough to exceed formation pressure in order to control the well while perforating the casing and performing other routine completion operations. In the case of a well filled with drilling mud when the casing is perforated, the drilling mud flows into the perforations because of the pressure diflerential existing between the interior of the casing and the formation. The perforations are thereby partially or completely plugged with mud and this plugging is aggravated by the heat and instantaneous pressure evolved by the propellant power in the case of 'bullet perforators and the high explosive .in the case of the jet or shaped charge perforators. Where chemical or punch-type perforators are employed, it is not uncommon for the drilling mud to lose water rapidly to the formation resulting in the drilling mud becoming dehydrated and forming plugs. Data are available which indicate thatsuch plugs whether formed by dehydration of the drilling mud by heat, and/or pressure or by loss of water to the formation are difiicult to remove by subsequent flow from the formation into the well bore and that the productivity of a perforating interval in a producing formation is significantly reduced. Field completion attempts of wells indicate that such plugging of the perforations may cause formations to be tested as being non-productive and thereby condemned when actually the formation may contain economically producible oil or gas. Thus it is clear that the problem of plugging of perforations is serious and is a source of expense in well completions and of erroneous conclusions in exploratory work which may cause major hydrocarbon reserves to remain undiscovered.

Another problem which exists .in perforating wells is that it is necessary to provide control of the well during ICC perforations. This is accordingly accomplished by maintaining a hydrostatic column which exerts a pressure greater than the formation pressure exposed when the casing is perforated. However, to provide a column having a suflicient hydrostatic pressure, it is necessary to add weighting agents such .as barite, sand, and other solid heavy materials to the fluid column maintained in the Well.

In accordance with the present invention the problem of plugging perforations in a well casing in which a column of drilling mud is maintained to control the Well'is eliminated by providing in the well casing adjacent the producing formation a zone from which the drilling mud is removed or excluded and then perforating the casing in the zone such that the drilling mud does not plug the perforations.

The present invention may, therefore, be briefly described as involving the replacing or displacing of the drilling mud at least in the region of the formation to be penetrated With a solids-free emulsion of an oily and aqueous medium containing a soluble weighting agent dissolved in one of the components of the emulsion, the emulsion being characterized by having a low fluid loss into the formation and having the property of displacing the drilling mud from the region of the casing to be perforated.

The emulsion employed in the practice of the present invention is used in the sense of the emulsions described in Sutheimns Introduction of Emulsions, Chemical Publishing Company, Inc., Brooklyn, New York, 1947, page 1 where an emulsion is described as follows: Emulsions are intimate mixtures of two immiscible liquids, one of them being dispersed in the other in the form of fine droplets. The emulsion of the present invention contains, dissolved in one of the components of the emulsion, a Weighting agent which is soluble in one of the components. The weighting agent may be soluble either in the aqueous phase or the oily phase of the emulsion. For example, the water phase may contain a water-soluble, inorganic weighting agent.

The oily phase may suitably be a hydrocarbon, such as crude petroleum and fractions thereof, such as gasoline, kerosene, gas oil, diesel oil, and the like. Other hydrocarbonaceous materials or derivatives of hydrocarbons, such as carbon tetrachloride and the like, may form all or part of the oily phase.

The emulsion may either be heavier or lighter than the drilling fluid and suitably may have a density in pounds per gallon in the range from about 7 to about 18 pounds per gallon. The emulsion where lighter than the drilling fluid may have a suitable viscosity suflicient to displace the drilling mud and to prevent displacement thereof by the drilling mud. For example, the viscosity .of the emulsion replacing the drilling mud may range from about to about 4000 centipoises at 60 F.

It is to be noted that, in allcases, the soluble weighting agents of Tables I and II have specific gravities greater than that of water.

The emulsion employed in the present invention will suitably consist essentially of from about 5% to about by volume of water With'a. preferred range from about 20 torabout 80% by volume of water, from about 5% to about'95% of oil with a preferred range from about 20 to 80 volume percent of oil, and an emulsifying agent of the'type illustrated in an amount in the range from about 0.5 to about .20 grams per ml. of the external phase of the emulsion. The weight percent of the inorganic weightingagent or salt dissolved in the aqueous 1 or water phase-will range from 1% up .to about saturation, while the weight percent of organic compound or weighting agent dissolved in the oil phase will range from 1% up to about saturation. Thefilm-strengtheningagent of the type illustrated where required may range from about 0.1 to about 1.0 grams per 100ml; of the aqueous phase of the emulsion. It is to be understood, of course, that when a weighting agent is used in the aqueous phase one ordinarilywillnot be required in the oil phase' or vice versa although it is contemplated that, under some circumstancesboth phases may contain a weighting agent merals 12 13, and 14. Intervals 12 and 14 may be provirtue of the emulsion 23 being free of solid materials ductive of hydrocarbons, such as oil and gas, while inand having the characteristics of low fluid loss and further terval 13 may be non-productive. A casing 15 is arranged the characteristic of displacing driling mud, the perforain the wellbore 11 and is cemented in place with primary tions 27 are clean and hydrocarbons may be easily procement 16. A tubing 17 is arranged in the casing 15 with duced from the formation 12. its lower open end 18 arranged above the uppermost of Thereafter, as illustrated in Fig. 4, the perforator 25 the intervals 12, 13, and 14. is withdrawn on wire line 26 and a second tubular mem- The casing 15 is shown fille'd with drilling mud indiber, such as 28, which is similar to tubular member 20 cated by the numeral 19. except of greater length, is lowered on wire line 26 en- In Fig. 2 a tubular extension member 20 is lowered gaged to fishing head 29 and landed and supported in through the tubing 17 by means of a wire line, not shown, landing nipple 21. Thereafter the drilling mud 19, both and landed, supported, and suspended in a landing nipple, above and below the emulsion 23 may be circulated out 21 which may be a landing nipple of the type illustrated in of the well by flowing a washing liquid, such as water, the Composite Catalog of Oil Field and Pipeline Equipindicated generally by the numeral 30, down the annulus t, 19th diti 1952-1953, page 4063, Th t b l between the tubing 17 and the casing 15. However, it is extension member is suitably provided with a sealing to be understood that the emulsion may be used as the means to seal between the upper end 22 of the tubular Washing liquid to remove the drilling mud by circulating memb 20 a d th tubing 17, the emulsion down the annulus betweenthe tubing 17 In accordance with the present invention at least a porand th C s g 15- The Washing liquid causes the mud tion of the drilling mud 19 is replaced by a body 23 of 20 and emulsion to be flowed up through the tubular extenemulsion such as illustrated by pumping the emulsion 23 i m r 28 and thence upwardly through the tubing d th t bi 17 d out th 1 15 1 t n i member 17. Production is then had from the clean perforations 20 to provide a sandwich of emulsign 23 arranged be- 27 into the casing 15. The tubular extension member 28 tween the displaced bodies of drilling mud 19, as shown y be Withdrawn and Production Continued through the in Fig. 2. After the emulsion 23 has been placed, as 25 p end 18 01H}!e tubing ho i Fi 2, th tubular extension member 20 i In order to illustrate the present invention further, a trieved from the tubing 17 by engaging a suitable wire number of compositions of emulsions in accordance with li fi hi l, not h i h h fi hi k 24 f the present invention were made. In these operations, the extension 20 and retrieving the tubular member 20 emulsions Were P p y miXiHg y materials th from the tubing 17. Thereafter a gun perforator, such Water containing inorganic Salts as Weighting agentsas 25, which may be a tubing gun perforator or one of Various amounts of the several emulsifying agents were the shaped charge type or a punch type or chemical peradded and the mixtures emulsified in a high speed agitator. forator or one which perforates casing by mechanical The Properties of the emulsion, Such as density and API means is lowered through the tubing 17 by a wire line 26 fluid 108s, are also givenand is inserted and/or located in the emulsion 23. The The composition and properties of emulsion prepared perforator 25 is then operated or fired to form a plurality h Sodium Chloride as a Soluble Weighting agent are of perforations 27 in the formation or interval 12. By presented in Table III: 1

9 It will be noted from an examination of the date in Table V that the emulsions had low fluid losses and had densities ranging from 10 to nearly 13 pounds per gallon. Additional emulsions were prepared with zinc chloride as the weighting agents. The composition and properties of these emulsions are presented in Table VI:

from about 180 F. to 190 F. and the properties thereof determined after varying periods of exposure to the aforesaid temperatures. The composition and the filtration rate of the selected emulsions before and after exposure to the high temperatures are presented in Table VII:

Examination of the :data in Table'VII shows that the emulsions are quite stable and retain their property of low fluid loss even after extended periods of exposure to high temperature. This importantin well completion -.In,order to illustrate the invention-further, several perforating targets were prepared by cementing 10" by 3V2" Berea outcrop cores into 4%" casing. A steel plate was provided atone end to simulate well perforating bmethods i p fi range indicated m 5 conditions. Emulsions were prepared in accordance with e encoun ere in we s t a i the presentomvention and placed in the perforatmgchams gg g g g g zi g zi film'strengtb her and the targets and chambers were maintained at g g p y p 180 F. for 24 hours. A pressure diilerential of 500 t TABLE VIII ounds er s uare inch was laced across the target with P P P lnfqrmaflon film'sfrengthemng flgents S911 I" p .the pressure differential into the core and then the target of emulsw was perforated'by firing into the target. After allowing Trade Designation Chemical Composition Manufacturer F? W }h vaned from about to about 24 hours, the target was backfiowed to determine if any moon SPF A Summated polymer 1 Mgnsamo Chemical .15breakdown-pressure. was required. The final permeabilompany. Driscose Sodium carboxymethyl Hercules Powder Comof the Perforated l j kerosene obtamed and M1 0M0 cegul ose pagy. 7 compared to the targets lfilllal permeability before perit sg d 58: forating. In two of the cases the target perforations were 7 o. o. t

A ras 0f0.7 sodium carboxymethvlgro p ap y g g s u t, core. The results'also show that acid zmg the cores'1ntzlgloslied form of CMC-7O H, viscosity of 1% solution at 25 0.: 1300 creases i i Permeabqity' In one cast? g p g g Average of p'g q p carboxymethyl oup per an ydmafter acidization was higher than the original permeab lity gucose unmexfm of the core. -Observations on the perforated targets.in-

1 Table slmllal t0 Y but glves the chem? dicated that a very small pressure dilferential initiated P si yin agents m l fiOW- v the Several Ples: The fact that only a small pressure difierential was AB required to initiate flow through the perforations is con- T LE IX 40 sidered very lmportant. This means, that 111 a well all Information m fi 1 p p of perforations made in an emulsion perforating medium emulslons will open to flow. This is not the case where mud is used as a perforating medium. Trade pesigna- Chemical Composition Class of Manufacturer It will be seen from the foregoing several examples Agent that an improved perforating technique has been provided which allows clean perforations to be formed by i span sorb tan monooleate non Ionic igi gifi" perforatmg with a clean emulsion whlch 1s free of sohds. fi a r gw a g q g gfi gg- Furthermore, it has been possible to provide emulsions p g which have the property of displacing the drilling fluid.

Reflex 30 g fi i gg a ffi gg g without being displaced by the drilling fluid. In other resin acidg words, an emulsion which has an inorganic weighting Tween g agentisprovided which is free of solids and is of sulfi- Brij a0 Pollyoiclyiathylene Iauryl Do. cient weight and/or viscosity to displace the drilling mud.

formation. Washing and circulating operations are exemplary of uses where the emulsion perforating fluids may be employed.

The present invention is, therefore, of considerable utility and advantage in perforating wells.

The nature and objects of the present invention having been completely described and illustrated, what we wish to claim as new and useful and to secure by Letters Patent is:

1. A method for completing a well penetrating a subsurface earth formation while maintaining said well under control with a fluid column having a hydrostatic pressure in excess of formation pressure which comprises replacing only a portion of the drilling fluid in a casing in said well with a solids-free heat stable emulsion which consists of from about to about 95 by volume of an aqueous medium, from about 5% to about 95 by volume of an oily medium, a weighting agent in an amount within the range between 1% to about saturation soluble and dissolved in at least one of said media, said weighting agent having a specific gravity greater than the specific gravity of the aqueous medium, and an emulsifying agent selected from the group consisting of sorbitan monooleate, sorbitan sesqui-oleate, polyoxyethylene sorbitan mono-stearate, polyoxyethylene lauryl alcohol, sodium lignosulfonate, polyoxyethylene sorbitol tetraoleate, polyoxyethylene sorbitol monolaurate, unpurified sorbitan mono-oleate, sorbitan monolaurate, and polyoxyethylene esters of mixed fatty and resin acids in an amount within the range between 0.2 and about 20 grams per ml. of the external phase of the emulsion, inserting a perforator in the emulsion, and operating the perforator in the emulsion to form clean perforations in said casing, said emulsion having a weight in the range from about 7 to about 18 pounds per gallon, being heat stable, and having a low fluid loss at high well temperatures whereby said earth formation is protected from damage by contact with fluids which might plug or reduce the permeability or porosity of the formation.

2. A method in accordance with claim 1 in which the soluble weighting agent is dissolved in the aqueous medium.

3. A method in accordance with claim 1 in which the soluble weighting agent is dissolved in the oily medium.

4. A method in accordance with claim 1 in which the soluble weighting agent is an inorganic metal salt dissolved in the aqueous medium.

5. A method in accordance with claim 1 in which an organic weighting agent having a specific gravity in the from about 0.1 to about 10 grams per 100 ml. of the aqueous phase of the emulsion.

7. A heat stable well completion composition having a weight in the range from about 7 to about 18 pounds per gallon which consists of a solids-free emulsion containing from about 5% to about by volume of an aqueous medium, from about 5% to about 95 by volume of an oily medium, a weighting agent having a specific gravity greater than the specific gravity of the aqueous medium in an amount within the range between 1% by weight to about saturation dissolved in at least one of said media, and an emulsifying agent selected from the group consisting of sorbitan mono-oleate, sorbitan sesquioleate, polyoxyethylene sorbitan mono-stearate, polyoxyethylene lauryl alcohol, sodium lignosulfonate, polyoxyethylene sorbitol tetraoleate, polyoxyethylene sorbitol monolaurate, unpurified sorbitan mono-oleate, sorbitan monolaurate, and poly'oxyethylene esters of mixed fatty and resin acids in an amount in the range between 0.2 and about 20 grams per ml. of the external phase of the emulsion, said composition being heat. stable and having a low fluid loss at high well temperatures.

8. A composition in accordance with claim 7 in which the emulsion contains a film-strengthening agent in an amount in the range from about 0.1 to about 10 grams per 100 m1. of the aqueous medium of the emulsion.

9. A composition in accordance with claim 7 in which the weighting agent is dissolved in the aqueous medium.

10. A composition in accordance with claim 7 in which the weighting agent is dissolved in the oily medium.

11. A composition in accordance with claim 7 in which the oily medium contains carbon tetrachloride.

12. A composition in accordance with claim 7 in which sodium chloride is the weighting agent dissolved in the aqueous phase.

13. A composition in accordance with claim 7 in which calcium chloride is the weighting agent dissolved in the aqueous phase.

14. A composition in accordance with claim 7 in which potassium carbonate is the weighting agent dissolved in the aqueous phase.

15. A composition in accordance with claim 7 in which zinc chloride is the weighting agent dissolved in the aqueous phase.